1. Technical Field
The present invention relates to a transistor of group-III nitride-based compound semiconductor used as a power electronic device or a high-frequency amplifying device, and a method of manufacturing the transistor.
2. Related Art
A wide bandgap semiconductor can be exemplified by group-III nitride-based compound semiconductor, and has high breakdown voltage, high electron mobility, and high thermal conductivity. Therefore, such a wide bandgap semiconductor is extremely useful as a material in semiconductor devices used in ways that involve high power, high frequency, or high temperature environments. For example, a field effect transistor (FET) having an AlGaN/GaN heterojunction structure is polarized due to the Piezo effect, causing a 2-dimensional electron gas (2DEG) to be formed at the interface. The 2DEG has high electron mobility and high carrier density, and therefore this type of FET can be expected for use as a power switching device with low ON-resistance and high-speed switching characteristics.
Furthermore, it is desirable that the switching element have a fail safe for preventing current from flowing when there is a problem. In other words, the switching element is required to perform a normally-off operation. A metal insulator semiconductor FET (MISFET) is known as a device having such a characteristic.
FIG. 12 shows an example of a conventional FET using GaN. A FET 900 shown in FIG. 12 includes a buffer layer 12 and an underlayer 13 of undoped AlGaN on a substrate 11 of, for example, sapphire. In the FET 900, an electron supplying layer 14 of n-type AlGaN and an electron transit layer 15 of n-type GaN are sequentially grown on the underlayer 13. A gate electrode 17 is formed above the electron transit layer 15, with an insulating film 16 interposed therebetween. The insulating film 16 is formed of a first insulating film 16a of AlN and a second insulating film 16b of SiO2 sequentially on the electron transit layer 15. A source electrode 18 and a drain electrode 19 are formed on the first insulating film 16a. 
In the FET 900, a leak current can be suppressed due to the second insulating film 16b, as described in Patent Document 1.    Patent Document 1: Japanese Patent Application Laid-open No. 2000-252458
However, the FET 900 described in Patent Document 1 has the following problem. Since the source electrode 18 and the drain electrode 19 are formed on the first insulating film 16a of AlN, the FET 900 has high contact resistance and high ON-resistance. Furthermore, it is necessary to lower the impurity concentration of the electron transit layer 15 in order to achieve a normally-off operation. When the impurity concentration of the electron transit layer 15 is lowered, however, the carrier mobility decreases.
The present invention has been achieved in view of the above aspects, and it is an object of the present invention to provide a normally-off FET that has high mobility and low ON-resistance.